zsync is a file transfer program. It allows you to download a file
from a remote server, where you have a copy of an older version of
the file on your computer already. zsync downloads only the new
parts of the file.

* Client-side rsync - zsync uses the rsync algorithm, but runs it
on the client side, thus avoiding the high server load associated
with rsync.

* Rsync over HTTP - zsync provides transfers that are nearly as
efficient as rsync -z or cvsup, without the need to run a special
server application. All that is needed is an HTTP/1.1-compliant web
server. So it works through firewalls and on shared hosting accounts,
and gives less security worries.

* Handling for compressed files - rsync is ineffective on compressed
files, unless they are compressed with a patched version of gzip.
zsync has special handling for gzipped files, which enables update
transfers of files which are distributed in compressed form.
